One of oxide superconductors having compositions expressed by formula La.sub.2-X Sr.sub.X CaCu.sub.2 O.sub.6, which has a composition of La.sub.1.6 Sr.sub.0.4 CaCu.sub.2 O.sub.6, is manufactured by a known method. In the method, predetermined raw materials such as nitrate, oxalate, and oxide are mixed to form a mixture having a predetermined composition, and then the mixture is preliminarily burned in a flow of oxygen at 900.degree. C. for 40 hours. Thereafter, the mixture is sintered in the same atmosphere at 925.degree. C. for 72 hours. The resultant sintered body is received in a cooled quartz tube, and then sealed in an airtight manner. This tube is heated at 970.degree. C., thereby making a pressurized atmosphere of 20 atm (i.e., 2 MPa) by using thermal expansion of oxygen condensed at the time of cooling the tube. The sintered body is kept in the pressurized atmosphere for 48 hours, and then is cooled at 850.degree. C., 750.degree. C., 650.degree. C. and 500.degree. C., successively, in units of five hours. Through these steps, a superconductor having a critical temperature of 50K or more is obtained (see R. J. Cava, Nature Vol. 345 (1990), pp. 602-604).
In the above-described method of manufacturing a superconductor, only a single manufacturing condition is disclosed, under which steps from the preliminary burning to the treatment in a pressurized atmosphere are performed. Further, the composition which the method can make superconductive is limited to La.sub.1.6 Sr.sub.0.4 CaCu.sub.2 O.sub.6, and hence any other composition (i.e., wherein the value of the Sr content x differs from that of the former composition) cannot have superconductivity.
In addition, in a case where a superconductive ceramic is produced by the above method, at least as long as 223 hours are required for performing the steps from the preliminary burning to the treatment in a pressurized atmosphere. Also, since a quartz tube is used for receiving a sintered body in an airtight manner, a batch of composite is greatly limited. Since the treatments are performed under only a single manufacturing condition, temperatures and pressures must be controlled with accuracy.